The present invention relates to a thermoplastic crosslinked product or heat-sensitive elastic adhesive which can be used with advantage as a sealing or adhesive having excellent workability and recyclability.
The sealant and adhesive in current use include hot-melt type and reactive sealant type. The hot melt type is predominantly composed of a thermoplastic elastomer and, for enhanced flexibility, contains a plasticizer such as mineral oil or paraffin oil. The hot melt type is superior in initial bond strength and workability but, because it is composed predominantly of a plasticized resin, is poor in heat resistance and weathering resistance. Moreover, because of its comparatively high plasticizer content, this adhesive is poor in antibleeding properties. The reactive sealant type is satisfactory in heat resistance and weathering resistance but poor in initial bond strength. Moreover, since it takes time for the adhesive to provide a necessary bond strength, the workability is unsatisfactory in some instances. Recently developed to overcome the shortcomings of said two types of adhesives is a reactive hot-melt material having the properties of a hot melt type and a reactive sealant type in one.
However, the reactive hot-melt material has the drawback that as the cure by reaction progresses, its thermoplasticity is lost to sacrifice workability and the material cannot be recycled any longer. The present invention has for its object to provide a thermoplastic crosslinked product or heat-sensitive elastic adhesive having the workability and recyclability of a hot-melt sealant and the rubber-like elasticity of a reactive type sealant.
As the catalyst which catalyzes the hydrolysis of a hydrolyzable silicon group or the condensation catalyst, tin compounds are well known, and for the purpose of curing polymers containing such silicon groups, tin compounds are generally employed. It is also known that the rubber-like properties such as stress relaxation and memory characteristics of the cured product differ according to whether the tin compound used is divalent or tetravalent. The cured product obtained by using a divalent tin compound is low in stress relaxation and high in memory characteristic, thus being close to the ideal elastomer, but the use of a tetravalent tin compound results in high stress relaxation and low memory characteristics. A relevant phenomenon observed in the crosslinked product obtained by using a tetravalent tin compound is the plastic deformation which occurs under prolonged loading. The inventors of the present invention found that this phenomenon unique to the crosslinked product obtained by curing a silicon group-containing polymer with a tetravalent tin compound is promoted by heating of the cured product and have developed the present invention.
The present invention, therefore, is directed to a thermoplastic crosslinked product obtainable by the crosslinking reaction of a composition comprising (A) a polymer having a silicon-containing group and (B) a tetravalent tin compound, said silicon-containing group having a hydrolyzable group bound to a silicon atom and capable of crosslinking through formation of a siloxane bond. The thermoplastic crosslinked product of the present invention shows thermoplasticity at a temperature lower than the thermal decomposition temperature of the main chain of the polymer.
In a further aspect, the present invention is directed to a heat-sensitive elastic adhesive comprising the thermoplastic crosslinked product of the present invention, that is to say a crosslinked product which is fluid under heating and shows rubber-like elasticity at room temperature (in the ordinary service temperature range).